In an atomic force microscope (AFM) a surface is imaged by scanning the surface with a cantilever. A sharp tip is formed at the end of the cantilever, and as the scanning takes place force interactions between the tip and the surface cause the cantilever to bend or deflect. Generally speaking, the AFM is operated either in the contact mode, in which the tip rides over the surface, or in the non-contact or attractive mode, in which the resonant frequency of a vibrating cantilever is measured with the tip of the cantilever positioned very near the surface. Variations in a gradient of a force between the tip and the surface (e.g., the van der Waals force) cause the resonant frequency of the cantilever to vary as the distance between the tip and the surface changes. In either mode of operation a negative feedback system is used to adjust the distance between the cantilever and the surface so as to maintain the measured parameter (bending or resonant frequency) at a constant, and an error signal in the feedback system is often used to generate an image of the surface. Depending on the properties of the tip and sample and subtleties of the measurement, it is possible to measure a wide range of surface properties, including topography, friction, magnetism, and electrical charge.
AFMs have also been used to modify surfaces, in substitution for conventional lithography systems. Whereas in conventional photolithography systems, resolution is limited by the wavelength of the light that is used to expose the resist, the resolution of an AFM is primarily limited by the sharpness of the tip. For example, hydrogen-passivated single-crystal silicon has been locally oxidized by biasing the tip relative to the silicon. Modifications made by electrical processes have been used to fabricate small-scale electrical devices such as transistors.
Whether the AFM is operating in the contact or non-contact mode, its resolution power depends on its ability to accurately detect the bending or deflection of the cantilever. There are several known ways of doing this. One of the most widely used techniques, described in U.S. Pat. No. 5,144,833, is to direct a light beam against a mirror surface on the back of the cantilever and to sense the location of the reflected beam with a position sensitive photodetector (PSPD). The PSPD normally consists of a pair of adjacent light detectors connected to the inputs of a differential amplifier. When the cantilever is in its equilibrium condition the reflected beam strikes both detectors equally, and the differential amplifier delivers a zero output. The noise generated by the detectors is additive. Since the output signal is a difference signal, the signal-to-noise ratio may be unacceptably low at small tip displacements.
Another known technique is to form a piezoresistive element in the cantilever and to detect the change in resistance of the piezoresistive element as the cantilever bends. While this method is easy to use, in general it is not as sensitive as the optical technique. Other known techniques use tunneling, capacitive and piezoelectric effects.
While the known techniques of detecting the deflection of the cantilever have allowed images to be generated down to the atomic scale, greater resolution could be obtained if the deflection of the cantilever could be detected with even greater precision. Moreover, since arrays of cantilevers are being used to form mask layers and otherwise modify surfaces in semiconductor fabrication and micro-machining operations, it would be particularly useful if a highly accurate system of detecting the deflection of individual cantilevers in an array could be devised.